1. Field of the Invention
This invention relates to a process of forming a trench or a deep hole by dry etching, in a semiconductor substrate, which is particularly useful for manufacturing an article made from Si, such as semiconductor DRAM device.
2. Description of the Related Art
Trenches formed by selectively digging down through a surface of a semiconductor are necessary to realize a high integration and high voltage withstanding properties in a semiconductor device. The configuration of each of the trenches must have a side wall which is a little inclined to form a taper, has a smooth surface, and has a round bottom as shown in FIG. 1A, from a view point of the manufacturing process and the characteristics of semiconductor devices. Thus, establishment of a technique for forming a trench having a good configuration is required. In FIG. 1A, 101 is silicon substrate and 102 is a side wall angle. Undesirable configurations are shown in FIG. 1B through FIG. 1H.
Heretofore, dry etching of Si for trenches (the same with deep holes, hereinafter only trenches being referred to) and others are conducted generally by using a fluorine-containing gas (hereinafter referred to as "F system gas"), a chlorine containing-gas (hereinafter referred to as "Cl system gas"), a bromine-containing gas (hereinafter referred to as "Br system gas"), etc., as disclosed in U.S. Pat. Nos. 4,226,665 and 4,208,241. Generally, forming a trench is conducted, as shown in FIG. 2A through FIG. 2C, by forming an insulating mask SiO.sub.2 202 over the Si substrate 201 (FIG. 1A), followed by forming a pattern in the mask, and then conduct etching. Here, a description of further processes for forming integrated circuits is eliminated.
At present, forming of trenches can be performed by an RIE (Reactive Ion Etching) device in an atmosphere of the Br system gas, F system gas, Cl system gas etc. However, a reliable and stable method as is applicable to commercial production is not yet known. Among the known processes, there is a report wherein the control of the side wall configuration of the trench can be attained by mixing etching gas SiCl.sub.4 and an inert gas, and carrying out dry etching with the gas pressure thereof being controlled.
During forming of a trench, not only the Si substrate 201 is etched, but also the SiO.sub.2 mask 202 is partially etched and becomes thin, as shown in FIG. 2C. The difficulty of mask diminishing is shown by "selectivity ratio against SiO.sub.2 ". The selectivity ratio against SiO.sub.2 is defined as a ratio of etching speed of semiconductor to etching speed of SiO.sub.2. The larger the value thereof, the deeper the trench can be made. This is because at the place where the selectivity ratio against SiO.sub.2 is small, the mask disappears before the trench is etched enough, and the Si of substrate which was under the SiO.sub.2 is eroded. Therefore, an etching gas having a large selectivity ratio against SiO.sub.2 is required. However, F system gas has generally low selectivity ratio against SiO.sub.2, and has a problem that it tends to cause isotropic etching wherein an etching reaction proceeds in all directions.
Further, as semiconductor devices become various and highly efficient, inhibition of the formation of etching residue is required in the trench formation in order to raise the production yield. The mechanism of forming etching residue is shown in FIGS. 4A and 4B. On a bared portion of Si 403 in a mask pattern before etching as shown in FIG. 4A, dusts or particles 404 adhere, or fragments of mask or reaction products, formed during the etching, adhere. Thus normal etching is inhibited and finally projections 405 as shown in FIG. 4B are formed, which are called black Si or Si black, which causes problems of decrease of yield etc. Generally, F system gas, Cl system gas, and Br system gas, in this order, increase the amount of black Si. In the above order, the latter gives a more ordered side wall configuration.
Prior art, as disclosed in U.S. Pat. No. 4,450,042, provides a general process for etching Si perpendicularly. This process, however, gives trenches at most 5 .mu.m deep, and if one tries to etch more deeply, one cannot necessarily form satisfactory trenches having stable configurations and high accuracy, because the configurations of trenches lack uniformity, or if uniform configurations are pursued, much residue is formed. In some cases a mixture of gases are used in order to take advantage of various etching gases. However an etching process is not yet proposed which solves all of the problems at the same time, to provide a commercial production system.
U.S. Pat. No. 4,784,720 discloses a process for controlling the trench configuration by selectively adhering reaction products onto the side walls of the trenches. However, the process of controlling the configuration by selectively adhering the etching reaction product is apt to produce etching residues. Since the residues, when they are left as they are, decrease yield, due to leakage and poor voltage breakdown properties, a further process is required to remove them. The patent also describes mixing SiCl.sub.4 and an inert gas and that the side wall configurations of trenches can be controlled by the gas pressure. However, the patent does not describe the inert gas clearly or mention other gases.